Apparatus and method for preparing and discharging an airstream laden with subliminable solid particles (for example, dry ice). The term xe2x80x9csubliminablexe2x80x9d as used herein includes xe2x80x9cevaporablexe2x80x9d.
Abrading and cleaning a surface by an air blast with entrained solid particles of a subliminable material is well-known. Dry ice is the best known substance for this purpose. Evaporable substances are included herein as subliminable, although subliminable materials are to be preferred. Among its advantages is the fact that in contrast to silica sand, it simply disappears after it has struck the surface to be cleaned, and its properties on impact are much more forgiving to the impacted surfaces.
The art includes many examples of apparatus which reduce blocks of solid carbon dioxide to particles of useful size to meter previously made particles of dry ice and then entrain them in an airstream which exits through a nozzle to impact a surface. The seeming simplicity of entraining particles of dry ice in an airstream is confounded by the physical properties of the product itself.
The dry ice is very cold, and as such chills everything it touches and closely approaches. The particles will readily aggregate into large lumps, with a different impact effect than the individual particles would have. Even worse, their aggregates tend to clog conventional metering and feeding mechanisms by freezing up in their chilled structures.
These problems are well known to manufacturers and users of apparatus for these processes, and the art is replete with efforts, some of them by the inventor herein, to improve on the situation. In view of the fact that the process involves no more than the reduction of a solid body of carbon dioxide into particles, and then transporting the particles to and through a nozzle, it is surprising that there still exists the need for further improvements to systems which already have been carefully and cleverly devised. However, such is in fact the situation.
Reduction of a larger body of solid carbon dioxide into particles and metering of particles is reasonably well developed. For example, see Opel et al U.S. Pat. No. 5,520,572. The problem arises in feeding these particles into a high pressure airstream, with a uniform and properly proportioned content of the particles, always available on demand, which is often intermittent and variable.
For this purpose some type of proportionalizing air lock equipment is needed. Known devices try to perform both functions and include moving elements which have cavities to receive the particles, and which function to supply them at an agreed rate. For example, push-pull plates and rotary star wheels are known whose cavities arrive at a supply station with a known frequency and discharge their contents at that frequency.
The customary approach to this requirement is to adjust the rate of supply of particles by fully filling the cavities in a feed mechanism, and then varying the rate of supply by adjusting the frequency at which the cavities arrive at their junction with the airstream. This is a satisfactory arrangement, but only within surprisingly narrow limits. As the demand for particles decreases, for example when a lesser flow is defined by the outlet nozzle, the filled cavities will discharge with a lesser frequency. This leads to a pulsating flow of particles at the nozzle which often does not produce a suitably uniform stream of particles.
Worse still, as stated above, at these slower rates the particles in the cavities can congeal to form larger agglomerates. This is serious enough, but when the wheel turns too slowly or stops with the cavities full, the particles can freeze in the cavity, and cannot be blown into the stream at all. Occurrences are frequent when the system must be shut down and the cavities cleared out before the system can be started again.
It is an object of this invention to provide apparatus and method to meter particles and entrain them into a pressurized airstream of any velocity and which also may demand more or fewer particles per unit of time, and more or less volume of air which airstream has only minimal fluctuations in the rate of particle presence at the nozzle.
It is another object of this invention to provide apparatus and method in which the particles do not tend to aggregate or to plug up the system or any part of it, especially in its air lock through which high pressure air is introduced into the stream.
A system according to this invention includes a source of dry ice particles, an adjustably variable-rate metering element, an air lock element, an adjustably regulable air supply, an air lock element receiving and combining both particles and air, a hose receiving a combined stream of air and particles from the air lock, and a nozzle discharging the particle-combining stream from the hose.
According to a preferred but optional feature of the invention, the metering element is a rotary auger, and the air lock element includes a rotor pierced by a plurality of separate passages disposed as a ring around the center of rotation of the rotor.
According to another preferred but optional feature of the invention the metering element and air lock element are separately controlled, such that the air lock element is in operation at all times when the metering element is operating and continues to operate after the metering element stops, whereby to clear the air lock element of any residual particles to prevent clogging.
According to yet another feature of the invention, the rate of rotation of the air lock element is independent of the feed rate from the metering element.
The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: